In a pulverized coal firing boiler, a combustion apparatus injects a mixture of pulverized coal and air into a furnace through a mixture feeding pipe. The mixture injected is ignited so as to form a flame in the furnace. As disclosed in United States Patent and Trademark Office U.S. Pat. No. 4,545,307, a radially outwardly flared flame maintaining ring is provided at an end of the mixture feeding pipe. Vortices of the mixture are formed along the flame maintaining ring so that the pulverized coal is concentrated in a vicinity of the flame maintaining ring. As a result an ignition takes place from the end portion of the mixture feeding pipe located in the furnace to form a high temperature strong reduction flame, thereby making it possible to suppress the generation of nitrogen oxides (NOx).
Disadvantages of the above construction reside in the fact that the flame maintaining ring gets covered with ashes and is maintained under a reduced atmosphere and, further, is exposed to high temperatures due to radiant heat from the furnace. These conditions may cause a burn out of the flame maintaining ring or, when the operation is not proper, a formation of slag on the flame maintaining ring, that is, promotion of the slagging, under certain circumstances. The burn out of the flame maintaining ring or the formation of slag results in the deterioration of the effect of the flame maintaining ring, an increase of the amount of NOx or malfunctioning of the apparatus.